fix: gate M4A to FLAC conversion on a codec probe in every branch

The SAF and local post-download branches used to rush an ffmpeg 'M4A to FLAC' remux whenever the output extension was .flac, which silently upscaled AAC or EAC3 streams into a lossless container. Each branch now mirrors the native worker by probing the primary audio codec before converting: lossless sources (and true FLAC-in-MP4 files) stay in their native container with the right extension, while genuine ALAC/WAV payloads still get remuxed.

Add an outputExt field to DownloadRequestPayload so the Go backend always knows the user-requested container, and use it together with _shouldRequestContainerConversion to pick the right behaviour for shouldPreserveNativeM4a and the Kotlin finalizer. Decryption descriptors no longer force M4A preservation on their own; the codec probe already makes that call correctly.

android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt

@@ -517,8 +517,9 @@ object NativeDownloadFinalizer {
     ) {
         if (requestQuality(input) == "HIGH" || outputExt(input) != ".flac") return
         val requestedDecryptionExt = requestedDecryptionOutputExt(input)
-        if (requestedDecryptionExt.isNotBlank() && requestedDecryptionExt != ".flac") return
-        val mayNeedContainerConversion = shouldForceContainerConversion(input, state) ||
+        val forceContainerConversion = shouldForceContainerConversion(input, state)
+        if (!forceContainerConversion && requestedDecryptionExt.isNotBlank() && requestedDecryptionExt != ".flac") return
+        val mayNeedContainerConversion = forceContainerConversion ||
             looksLikeM4a(state.filePath, state.fileName) ||
             state.filePath.startsWith("content://")
         if (!mayNeedContainerConversion) return